Presence of Colistin- and Tigecycline-Resistant ST29 in Municipal Wastewater Influents in Japan.

The aim of this study was to investigate the presence of colistin- and/or tigecycline-resistant spp. in influents from four wastewater treatment plants (WWTPs), which partly reflect the gut microbiome of human populations. Colistin- and tigecycline-resistant isolates (K30/ST29) were detected four times from the WWTP A during a period of 3 months. Disruptions of the and genes by IS and IS, respectively, were identified in those four isolates. They also shared the IncL/M 86,197-bp plasmids carrying a and Tn-associated [IS-------IS-IS--IS-(E)-(E)-IS]. Those isolates formed a distinct cluster within wgMLST clusters of ST29 K30 public reference strains of human origin and were unique due to harboring of Tn-like mercury resistance operon transposons in addition to silver, copper, and arsenic resistance determinants. Five strains with different STs and 1 strain, exhibiting colistin resistance, were detected in WWTPs B, C, and D. For these isolates, disruptions of by IS (three isolates) or IS (one isolate), insertion of IS in the promoter region (one isolate), and inactivation of MgrB by a nonsense mutation (one isolate) were identified. Close monitoring of these -negative colistin- and/or tigecycline-resistant bacteria in wastewater influents is imperative to avoid further limiting of treatment options.